LCD display devices in monitors, TVs, computers, mobile devices, wireless devices and so on typically have a relatively slow response time while switching a pixel from a grey level to another grey level. Generally, moving images have disturbed appearances leading to motion portrayal artifacts on the LCD display devices. The image needs to be rendered properly on the LCD display device in order to reduce such artifacts.
The slow response time of the LCD display devices is caused by the fact that, upon a frame change, it takes a couple of frame-times before a pixel reaches its intended transmission value due to the inherent slowness of the liquid crystal materials.
It is known that some LCD display drivers implement an overdrive technique. US20030156092 describes the implementation of such an overdrive technique. In this document a display driver hosting a frame memory as well as an operational unit controlling the display device is disclosed. The scope of this invention is in implementing overdrive.
Overdrive is a technique for writing a display data signal that is temporarily more emphasized than the display data signal corresponding to actual pixel transmission of the LCD display device. Due to this technique, the liquid crystal cell of the LCD display device reaches the intended transmission much faster. The overdrive technique thus improves the display performance of moving images on LCD display devices as it enhances the pixel response time.
This overdrive technique works by representing an incoming display data signal as a pixel drive voltage, which is greater than the required voltage of that pixel for better transmission. Similarly, whenever the pixel transmission needs to be decreased a lower pixel voltage is supplied.
This technique uses information of the display data signal of the previous frame, the display data signal of the current frame and an overdrive lookup table to calculate the corrected signal for overdrive. The signal that is overdrive corrected is then transmitted to the pixels of the LCD display device to display the corresponding image for the incoming display data.
The problem with the techniques discussed in the prior art is that refreshing the display data on the LCD display device requires a large frame-memory and leads to relatively high power consumption.
It is an object of the present invention to provide improved motion portrayal in particular to LCD display devices with relatively low power consumption.
The display driver according to the invention as specified in claim 1 has achieved this object. The driver comprises an embedded frame memory and an overdrive logic block, for moderating display data of a current frame received by the display driver by means of overdrive, wherein the overdrive logic block is arranged for reading data from and writing data to the embedded frame memory, and for using display data of a previous frame stored in the embedded frame memory for calculating overdrive display data of the current frame.
The embedded frame memory and the overdrive logic block are hosted within the display driver to achieve overdrive with no additional hardware. The overdrive logic block is used for reading data from and writing data to the embedded frame memory and also performs the calculations related of the pixel drive voltages that need overdrive. The display data of the previous frame is used by the overdrive logic block for calculating the overdrive correction to be applied to the incoming display data of the current frame. This mode of operation is referred to hereinafter as the indirect display mode or the internal timing mode.
A further embodiment is characterized in that the overdrive display data is calculated on alternating frames.
In a further embodiment overdrive correction factors are stored in an overdrive lookup table and are used for calculating the overdrive display data.
The said overdrive lookup table may be implemented using a read-only-memory (ROM), an electrically erasable programmable read-only-memory (EEPROM) or any other storage devices having a similar function. The overdrive logic block uses the overdrive lookup table to obtain the correction factor to be applied to the incoming display data signal of the current frame. These overdrive display data is thus preferably calculated from the overdrive display data of the previous frame stored in the memory, the incoming display data of the current frame, and an appropriate overdrive correction factor obtained from the lookup table.
Another preferred embodiment is characterized in that the embedded frame memory stores the overdrive display data for at least part of the current frame.
Overdrive must act on images and not on frames. Generally, in mobile devices the image refresh rate is very low. Therefore, in mobile applications frame rate up-conversion is often applied, by duplicating image data, leading to multiple frames containing the same image data.
A further embodiment of the invention is characterized in that the driver is further being arranged to operate in a frame rate up-conversion mode, wherein the embedded frame memory is used as a frame store for repeating the display data. Preferably frame rate up-conversion operates on static images.
Preferably, the driver operates in the frame rate up-conversion mode when the incoming display data comprises mainly static images such as background images and menus.
The driver can also be set to operate in a direct display mode when the incoming display data comprises mainly full screen moving images such as a video clip. In the direct display mode the embedded frame memory no longer stores the display data being displayed on the LCD panel, instead it may have different functions. An external control unit generates timing signals for controlling direct transmission of the display data to the LCD panel.
In a further preferred embodiment of the direct display mode, the embedded frame memory is a frame-delay FIFO for overdrive correction of the display data.
Yet another further embodiment is that the overdrive display data is calculated at least for a part of the display area representing a video window with moving images.
The advantage of storing part of a frame in the embedded frame memory is that only the video window needs to be refreshed in every frame, a static part of the frame is kept in the memory and can be refreshed less often. The embedded frame memory does store an entire frame however the image data for the video window is used for overdrive correction of the next image in the video window.
A further embodiment is that the driver is further being arranged to operate in an overlay mode, wherein the embedded frame memory is a frame overlay for mixing display data.
The overlay data is stored in the embedded frame memory. The overlay data such as a phone menu, is fetched from embedded frame memory and mixed with background display data, using a multiplexer or mixer, and displayed on the LCD panel. The multiplexer outputs both the background and incoming display data on the LCD panel, preferably in a predetermined ratio in the direct display mode.
In a further embodiment the driver comprises means for switching between different operational modes, such as the direct display mode with overdrive, overlay mode, the indirect display mode and frame-rate up-conversion mode.
Another preferred embodiment is characterized in that the overdrive display data enhances the response time of an LCD panel.
The overdrive pixel voltage enhances the voltage supplied to the pixels of an LCD display panel in order to speed up a change in the optical transmission of the pixels to be displayed on the LCD display device. The advantage of this is that the response time of the LCD display device is enhanced.
Another aspect of the invention is a LCD display device comprising a display driver as described in the above. Achieving overdrive and improving motion portrayal by the display driver in accordance with the invention improves efficiency of the LCD display device with little additional hardware and lesser power consumption.